Starship prompt fixes

[afrigon]

• show user@host only in ssh sessions (custom modules gated on $SSH_CONNECTION/$SSH_TTY — the builtin username module also fires on login-user misdetection and cannot be restricted)
• give the worktree module an actual icon (the config had a bare space as symbol) and hide the worktree name when it matches the branch, keeping the icon
• drop the "on " prefix from the branch module
• pin custom modules to sh: starship runs them with $STARSHIP_SHELL, and the multi-line command is not valid fish